Pergamon ?-errahedron Lerrers. Vol. 37. No. 44, pp. 8015-8018. 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved PII: SOO40-4039(96)018 13-8 0040.4039/96 $i5.00 + 0.00 Synthesis of 2-Lithio-1,3-Diselenanes and 2-Lithio-1,3-Dithianes by Se/Li Exchange : First Successful Trapping of Axially Oriented 2-Lithio-1,3- Diselenanes. Alain Krief* and Laurent Defrhre Department of Chemistry, Facult& Universitaires Notre-Dame de la Paix, 61 rue de Bruxelles B-5000, NAMUR (Belgium) Abstract : Conformationally rigid 2-methylseleno-1,3-diselenanes beanng the methylseleno moiety in axial position react with butyllithiums at low temperature (-130°C) to produce 2-lithio-1.3-diselenanes bearing the lithium in axial position. These organometallics are configurationally stable at -130°C and slowly epimerise at -78°C. Deuteration occurs exclusively from the axial side if carried out at -130°C. 2-Methylseleno-I .3-dithianes behave differently since epimerisation of 2-lithio-1,3-dithianes almost immediately takes place already at -130°C. C opyright 0 1996 Elsevier Science Ltd It is firmly established that metallation of 1,3-dithianes la1 and 1,3-diselenanes 1 b2 occurs regioselectively at C-2 on the equatorial rather than on the axial hydrogen. It has been also reported that reaction of the resulting 2-metallo-1,3- dithianes 2a and 2-metallo-1,3-diselenanes 2b with electrophiles leads to products 3’ bearing the Incoming group invariably on the equatorial position (Scheme 1). 1 v2 This therefore requires, in the case of 1” an isomerisation of the related organometallic 2” from the axial to the equatorial position leading to 2’ whatever is the nature and the bulkiness of the other substituent present at C-2. Trapping the 2-metallo-derivatives 2” bearing the metal in the axial position, prior to its isomerisation, has been for long a challenge but all the efforts directed towards this end have been up to now unsuccessfu1.l s2 Scheme 1